Solubility of amorphous Th(IV) hydroxide – application of LIBD to determine the solubility product and EXAFS for aqueous speciation

Karst-bauxite formation during the Great Oxidation Event indicated by dating of authigenic rutile and its thorium content

Aluminium (Al)-rich palaeosols-i.e., palaeobauxite deposits-should have formed in karst depressions in carbonate sequences as a result of acidic solutions from oxidative weathering of sulfide minerals during the Great Oxidation Event (GOE), but no GOE-related karst-palaeobauxite deposits have so far been recorded. Here, we report results of in situ uranium-lead (U-Pb) dating of detrital zircon and spatially associated rutile from a metamorphosed Al-rich rock within a dolomite sequence in the Quadrilátero Ferrífero (QF) of Minas Gerais, Brazil, known as the Gandarela Formation. Rutile grains are highly enriched in thorium (Th = 3-46 ppm; Th/U ratio = 0.3-3.7) and yielded an isochron, lower-intercept age of ca. 2.12 Ga, which coincides with the final phase of the GOE-i.e., the Lomagundi event. The rutile age represents either authigenic growth of TiO₂ enriched in Th, U and Pb during bauxite formation, or subsequent rutile crystallisation during metamorphic overprint. Both cases require an authigenic origin for the rutile. Its high Th contents can be used as a palaeoenvironmental indicator for decreased soil pH during the GOE. Our results also have implications for iron (Fe)-ore genesis in the QF. This study demonstrates that in situ U-Th-Pb-isotope analyses of rutile can place tight constraints on the age and nature of palaeosols.

Interlink between solubility, structure, surface and thermodynamics in the ThO2(s, hyd)–H2O(l) system

The impact of temperature on a freshly precipitated ThO2(am, hyd) solid phase was investigated using a combination of undersaturation solubility experiments and a multi-method approach for the characterization of the solid phase. XRD and EXAFS confirm that ageing of ThO2(am, hyd) at T = 80°C promotes a significant increase of the particle size and crystallinity. TG-DTA and XPS support that the ageing process is accompanied by an important decrease in the number of hydration waters/hydroxide groups in the original amorphous Th(IV) hydrous oxide. However, while clear differences between the structure of freshly precipitated ThO2(am, hyd) and aged samples were observed, the characterization methods used in this work are unable to resolve clear differences between solid phases aged for different time periods or at different pH values. Solubility experiments conducted at T = 22°C with fresh and aged Th(IV) solid phases show a systematic decrease in the solubility of the solid phases aged at T = 80°C. In contrast to the observations gained by solid phase characterization, the ageing time and ageing pH significantly affect the solubility measured at T = 22°C. These observations can be consistently explained considering a solubility control by the outermost surface of the ThO2(s, hyd) solid, which cannot be properly probed by any of the techniques considered in this work. Solubility data are used to derive the thermodynamic properties (log *K°s,0, ΔfG°m) of the investigated solid phases, and discussed in terms of particle size using the Schindler equation. These results provide new insights on the interlink between solubility, structure, surface and thermodynamics in the ThO2(s, hyd)–H2O(l) system, with special emphasis on the transformation of the amorphous hydrous/hydroxide solid phases into the thermodynamically stable crystalline oxides.

Accountancy for intrinsic colloids on thorium solubility: The fractionation of soluble species and the characterization of solubility limiting phase

The extensive hydrolysis of tetravalent actinides leads to polynuclear formations through oxygen bridging facilitating the formation of colloids as end products. The pH, ionic strength has phenomenal effects on Thorium colloids formation. The quantitative estimation of colloids facilitates the fraction of soluble fraction into ionic, polymeric and colloidal forms of thorium. The colloids accountability and precipitate characterization explains the discrepancies in estimated solubility limits. The supernatants of long equilibrated (∼3 years) saturated thorium solution under various pH (5- 11) and ionic strengths (0-3 M NaClO₄) were analysed by Inductively Coupled Plasma Mass Spectrometer (ICP-MS) and Ion Chromatography (IC) to determine total and ionic thorium respectively. Laser Induced Breakdown Detection (LIBD) was employed to determine the colloid size and concentrations. The precipitates were characterized by calorimetry and XRD to determine the solubility limiting phase. The results of pH, IC, ICP-MS, and LIBD measurements on the aged thorium samples are discussed with regard to the mechanism of the formation of thorium colloids. The results revealed the formation of colloids having particle size (10-40 nm) at concentrations (10⁹-10¹¹ particles/mL). The colloids accountancy resulted in estimated solubility products to 2-4 orders lower than their inclusion as soluble thorium. The soluble thorium was fractionated quantitatively into ionic, polymeric and colloidal forms of thorium. The precipitates formed are found to be semi amorphous.

Properties of the tetravalent actinide series in aqueous phase from a microscopic simulation self-consistent engine

In the context of nuclear fuel recycling and environmental issues, the understanding of the properties of radio-elements with various approaches remains a challenge regarding their dangerousness. Moreover, experimentally, some issues are also of importance; first, it is imperative to work at sufficiently high concentrations to reach the sensitivities of the analytical tools, however this condition often leads to precipitation for some of them; second, stabilizing specific oxidation states of some actinides remains a challenge, thus making it difficult to extract general trends across the actinide series. Complementary to experiments, modeling can be used to unbiasedly probe the actinide's properties in an aquatic environment and offers a predictive tool. We report the first molecular dynamics simulations based on homogeneously built force fields for the whole series of the tetravalent actinides in aqueous phase from ThIV to BkIV and including PuIV. The force fields used to model the interactions among the constituents include polarization and charge donation microscopic effects. They are built from a self-consistent iterative ab initio based engine that can be included in future developments as an element of a potential machine learning procedure devoted to generating accurate force fields. The comparison of our simulated hydrated actinide properties to available experimental data shows the model robustness and the relevance of our parameter assignment engine. Moreover, our simulated structural, dynamical and evolution of the hydration free energy data show that, apart from AmIV and CmIV, the actinide properties change progressively along the series.

Probing the local structure of nanoscale actinide oxides: a comparison between PuO2 and ThO2 nanoparticles rules out PuO2+x hypothesis

Actinide research at the nanoscale is gaining fundamental interest due to environmental and industrial issues. The knowledge of the local structure and speciation of actinide nanoparticles, which possibly exhibit specific physico-chemical properties in comparison to bulk materials, would help in a better and reliable description of their behaviour and reactivity. Herein, the synthesis and relevant characterization of PuO₂ and ThO₂ nanoparticles displayed as dispersed colloids, nanopowders, or nanostructured oxide powders allow to establish a clear relationship between the size of the nanocrystals constituting these oxides and their corresponding An(iv) local structure investigated by EXAFS spectroscopy. Particularly, the first oxygen shell of the probed An(iv) evidences an analogous behaviour for both Pu and Th oxides. This observation suggests that the often observed and controversial splitting of the Pu-O shell on the Fourier transformed EXAFS signal of the PuO₂ samples is attributed to a local structural disorder driven by a nanoparticle surface effect rather than to the presence of PuO_2+x species.

Homogeneous hydrolysis of thorium by thermal decomposition of urea

Thorium was precipitated homogeneously from a thorium nitrate solution by the thermal decomposition products of urea. The kinetics of the hydrolysis were studied at 90 and 100°C by pH measurement during the initial 5 h and the precipitation efficiencies of thorium and radium were measured over a 24 h period. Precipitation of the radium daughters was closely followed with the aim of co-precipitation of radium with thorium. The CO2 formed during urea decomposition dissolved in the solution, forming CO32− during the experiment upon reaching a sufficiently high pH level (>7). This allowed radium to co-precipitate partially, thus reducing the activity of the filtrate. After filtration or centrifugation, the precipitate is composed of nanocrystalline thorium dioxide (crystallite size ~10 nm), with weakly bound H2O and CO2.

Solubilities and solubility products of thorium hydroxide under moderate temperature conditions

The Th solubilities of the sample solutions that initially contained Th(OH)4(am) prepared by undersaturation and oversaturation methods in the pHcrange of 2.0–8.0 in a 0.5 M ionic strength solution of NaClO4and HClO4and stored at aging temperatures (Ta) of 298, 313 and 333 K were investigated in this study. After a certain period of time up to 40 weeks depending onTa, supernatants of the sample solutions were ultrafiltrated through 3 kDa membranes under the measurement temperature (Tm) of 298, 313 and 333 K. Size distributions of the colloidal species were investigated by ultrafiltration using membranes with different pore sizes ranging from 3 to 100 kDa, and the solid phases were examined by X-ray diffraction (XRD). The solubility of the sample solutions obtained after aging atTa=298 K using undersaturation method with continuous shaking was similar to those of dried precipitate of Th hydroxide. The solubilities obtained after aging atTa=313 and 333 K were lower than those atTa=298 K. The XRD spectra suggested that the crystallization of the solid phase proceeded under these elevated temperatures. The solubility of the sample solutions obtained after aging atTa=333 K using the oversaturation technique were similar to those prepared by undersaturation method and aged at the sameTa. A slight temperature dependence of the apparent solubilities on theTmwas observed in the sample solutions prepared by both methods. The solubility products$({K_{{\text{sp,}}{T_{\text{a}}}}}({T_{\text{m}}}))$after differentTaandTmwere determined from the solubility analysis. The observed increase in the formation constant$({K_{{\text{s,}}{T_{\text{a}}}}}({T_{\text{m}}}))$of Th4++(4+x)H2O(1)⇌Th(OH)4·xH2O(s,Ta)+4H+with increasingTmindicated that the reaction was endothermic. The enthalpy change$(\Delta_{r}H_{m\_ T_{\text{a}} \to {\text{cr}}}^{\circ} )$between the solid phases of Th(OH)4·xH2O(s,Ta) and ThO2(cr) suggested that the solid phase transformation from Th(OH)4·xH2O(s,Ta) to ThO2(cr) contains an endothermic process.

Aqueous chemistry of Ce(iv): estimations using actinide analogues

The prediction of cerium (Ce) aqueous speciation is relevant in many research fields. Indeed, Ce compounds are used for many industrial applications, which may require the control of Ce aqueous chemistry for their synthesis. The aquatic geochemistry of Ce is also of interest. Due to its growing industrial use and its release into the environment, Ce is now considered as an emerging contaminant. Cerium is also used as a proxy of (paleo)redox conditions due to the Ce(iv)/Ce(iii) redox transition. Finally, Ce(iv) is often presented as a relevant analogue of tetravalent actinides (An(iv)). In the present study, quantum chemical calculations were conducted to highlight the similarities between the structures of Ce(iv) and tetravalent actinide (An(iv); An = Th, Pa, U, Np, Pu) aqua-ions, especially Pu(iv). The current knowledge of An(iv) hydrolysis, solubility and colloid formation in water was briefly reviewed but important discrepancies were observed in the available data for Ce(iv). Therefore, new estimations of the hydrolysis constants of Ce(iv) and the solubility of Ce(iv)-(hydr)oxides are proposed, by analogy with Pu(iv). By plotting pH-Eh (Pourbaix) diagrams, we showed that the pH values corresponding to the onset of Ce(iv) species formation (i.e. Ce(iv)-(hydr)oxide or dissolved Ce(iv)) agreed with various experimental results. Although further experimental studies are required to obtain a more accurate thermodynamic database, the present work might yet help to predict more accurately the Ce chemical behavior in aqueous solution.

Retardation of uranium and thorium by a cementitious backfill developed for radioactive waste disposal

The solubility of uranium and thorium has been measured under the conditions anticipated in a cementitious, geological disposal facility for low and intermediate level radioactive waste. Similar solubilities were obtained for thorium in all media, comprising NaOH, Ca(OH)₂ and water equilibrated with a cement designed as repository backfill (NRVB, Nirex Reference Vault Backfill). In contrast, the solubility of U(VI) was one order of magnitude higher in NaOH than in the remaining solutions. The presence of cellulose degradation products (CDP) results in a comparable solubility increase for both elements. Extended X-ray Absorption Fine Structure (EXAFS) data suggest that the solubility-limiting phase for uranium corresponds to a becquerelite-type solid whereas thermodynamic modelling predicts a poorly crystalline, hydrated calcium uranate phase. The solubility-limiting phase for thorium was ThO₂ of intermediate crystallinity. No breakthrough of either uranium or thorium was observed in diffusion experiments involving NRVB after three years. Nevertheless, backscattering electron microscopy and microfocus X-ray fluorescence confirmed that uranium had penetrated about 40 μm into the cement, implying active diffusion governed by slow dissolution-precipitation kinetics. Precise identification of the uranium solid proved difficult, displaying characteristics of both calcium uranate and becquerelite.

Assessment of surface reactivity of thorium oxide in conditions close to chemical equilibrium by isotope exchange 229Th/232Th method

This work aims to assess the solubility and the surface reactivity of crystallized thorium at pH 3.0 in presence of three types of solids: synthesized powder at 1300°C, crushed kernel, and intact kernel. In this study, the kernel is composed by the core solid from high temperature reactors (HTR) sphere particles. The originality of this work consisted in following in a sequential order the kinetic of dissolution, the surface reactivity in presence of isotope tracer 229Th, and its desorption process. Long time experiments (634 days) allowed to get deeper understanding on the behavior of the surface reactivity in contact with the solution. Solubility values are ranging from 0.3×10−7 mol·L−1 to 3×10−7 mol·L−1 with a dissolution rate of 10−6–10−4 g·m−2 day−1. PHREEQC modeling showed that crystallized ThO2(cr, 20 nm) phase controls the equilibrium in solution. Isotope exchange between 229Th and 232Th indicated that well-crystallized phase exist as an inert surface regarding to the absence of exchange between surface solid and solution.

Hydrolysis of thorium(iv) at variable temperatures

Hydrolysis of Th(iv) was studied in tetraethylammonium perchlorate (0.10 mol kg(-1)) at variable temperatures (283-358 K) by potentiometry and microcalorimetry. Three hydrolysis reactions, mTh(4+) + nH2O = Thm(OH)n((4m-n)+) + nH(+), in which (n,m) = (2,2), (8,4), and (15,6), were invoked to describe the potentiometric and calorimetric data for solutions with the [hydroxide]/[Th(iv)] ratio ≤ 2. At higher ratios, the formation of (16,5) cannot be excluded. The hydrolysis constants, *β2,2, *β8,4, and *β15,6, increased by 3, 7, and 11 orders of magnitude, respectively, as the temperature was increased from 283 to 358 K. The enhancement is mainly due to the significant increase of the degree of ionization of water as the temperature rises. All three hydrolysis reactions are endothermic at 298 K, with enthalpies of (118 ± 4) kJ mol(-1), (236 ± 7) kJ mol(-1), and (554 ± 4) kJ mol(-1) for ΔH2,2, ΔH8,4, and ΔH15,6 respectively. The hydrolysis constants at infinite dilution have been obtained with the specific ion interaction approach. The applicability of three approaches for estimating the equilibrium constants at different temperatures, including the constant enthalpy approach, the constant heat capacity approach and the DQUANT equation was evaluated with the data from this work.

Oxyhydroxy Silicate Colloids: A New Type of Waterborne Actinide(IV) Colloids

At the near-neutral and reducing aquatic conditions expected in undisturbed ore deposits or in closed nuclear waste repositories, the actinides Th, U, Np, and Pu are primarily tetravalent. These tetravalent actinides (AnIV) are sparingly soluble in aquatic systems and, hence, are often assumed to be immobile. However, AnIV could become mobile if they occur as colloids. This review focuses on a new type of AnIV colloids, oxyhydroxy silicate colloids. We herein discuss the chemical characteristics of these colloids and the potential implication for their environmental behavior. The binary oxyhydroxy silicate colloids of AnIV could be potentially more mobile as a waterborne species than the well-known mono-component oxyhydroxide colloids.

First structural characterization of Pa(iv) in aqueous solution and quantum chemical investigations of the tetravalent actinides up to Bk(IV): the evidence of a curium break

More than a century after its discovery the structure of the Pa(4+) ion in acidic aqueous solution has been investigated for the first time experimentally and by quantum chemistry. The combined results of EXAFS data and quantum chemically optimized structures suggest that the Pa(4+) aqua ion has an average of nine water molecules in its first hydration sphere at a mean Pa-O distance of 2.43 Å. The data available for the early tetravalent actinide (An) elements from Th(4+) to Bk(4+) show that the An-O bonds have a pronounced electrostatic character, with bond distances following the same monotonic decreasing trend as the An(4+) ionic radii, with a decrease of the hydration number from nine to eight for the heaviest ions Cm(4+) and Bk(4+). Being the first open-shell tetravalent actinide, Pa(4+) features a coordination chemistry very similar to its successors. The electronic configuration of all open-shell systems corresponds to occupation of the valence 5f orbitals, without contribution from the 6d orbitals. Our results thus demonstrate that Pa(iv) resembles its early actinide neighbors.

A new fluorescent sensor for determination of thorium by thin film of 2-(acetyloxy)-N-(5-nitro-2-thiazolyl)-benzamide embedded in sol–gel matrix

A new “turn-off” fluorescent sensor for determination of thorium (Th(iv)) by thin film of 2-(acetyloxy)-N-(5-nitro-2-thiazolyl)-benzamide (L) embedded in sol–gel matrix was introduced.

Effect of anthropogenic organic complexants on the solubility of Ni, Th, U(IV) and U(VI)

The influence of anthropogenic organic complexants (citrate, EDTA and DTPA from 0.005 to 0.1M) on the solubility of nickel(II), thorium(IV) and uranium (U(IV) and U(VI)) has been studied. Experiments were carried out in 95%-saturated Ca(OH)2 solutions, representing the high pH conditions anticipated in the near field of a cementitious intermediate level radioactive waste repository. Results showed that Ni(II) solubility increased by 2-4 orders of magnitude in the presence of EDTA and DTPA and from 3 to 4 orders of magnitude in the case of citrate. Citrate had the greatest effect on the solubility of Th(IV) and U(IV)/(VI). XRD and SEM analyses indicate that the precipitates are largely amorphous; only in the case of Ni(II), is there some evidence of incipient crystallinity, in the form of Ni(OH)2 (theophrastite). A study of the effect of calcium suggests that U(VI) and Ni(II) may form metal-citrate-OH complexes stabilised by Ca(2+). Thermodynamic modelling underestimates the concentrations in solution in the presence of the ligands for all the elements considered here. Further investigation of the behaviour of organic ligands under hyperalkaline conditions is important because of the use of the thermodynamic constants in preparing the safety case for the geological disposal of radioactive wastes.

Colloid-borne forms of tetravalent actinides: a brief review

Tetravalent actinides, An(IV), are usually assumed to be little mobile in near-neutral environmental waters because of their low solubility. However, there are certain geochemical scenarios during which mobilization of An(IV) in a colloid-borne (waterborne) form cannot be ruled out. A compilation of colloid-borne forms of tetravalent actinides described so far for laboratory experiments together with several examples of An(IV) colloids observed in field experiments and real-world scenarios are given. They are intended to be a knowledge base and a tool for those who have to interpret actinide behavior under environmental conditions. Synthetic colloids containing structural An(IV) and synthetic colloids carrying adsorbed An(IV) are considered. Their behavior is compared with the behavior of An(IV) colloids observed after the intentional or unintentional release of actinides into the environment. A list of knowledge gaps as to the behavior of An(IV) colloids is provided and items which need further research are highlighted.

Adsorption of tetravalent thorium by geomedia

We measured the pH-dependent adsorption of Th(IV), an analogue for Pu(IV) and other tetravalent actinides, to two geomedia: goethite (α-FeOOH(s)) and a heterogeneous Fe-containing sand from the southeastern USA. The goal was to examine whether or not the Th(IV)-goethite adsorption data could be used to predict the adsorption of Th(IV) by the heterogeneous sand. In the absence of either geomedia, after forty-eight hours the measured pH-dependent “adsorption” was consistent with the solubility of solid amorphous ThO2(am, aged), despite the fact that ThO2(am, aged) is generally not formed until approximately seventy days. We concluded that ThO2(am, aged) was stabilized by precipitating on the walls of the reaction vessels. Ignoring this phenomenon could lead to experimental artifacts in Th(IV) adsorption studies. Thorium adsorption by both goethite and the sand was strongly pH dependent, with adsorption increasing sharply from pH ∼ 2 to pH ∼ 4. Two methods were utilized to predict the pH-dependent adsorption of Th(IV) by the sand using the Th(IV)-goethite adsorption data. Using the Fe content of the sand and the Th(IV)-goethite adsorption data, we were able to predict the maximum amount of Th(IV) adsorption by the sand within 78% of the actual value (i.e., an error of 22%). In contrast, on a surface-area-normalized basis, we were only able to predict the maximum adsorption of Th(IV) by the sand within a factor of two. These results have important implications to scaling and extrapolating the results of batch-scale tetravalent actinide adsorption studies with pure minerals to predict their field-scale adsorption by and transport in heterogeneous subsurface media.

Solubility of ThO2·xH2O(am) in the presence of gluconate

Thorium complexation with gluconate has been studied from solubility experiments at gluconate concentrations between 10−6 and 10−1 mol/l, with pHc between 9 and 13 and I=0.5 M (NaClO4). Solubility experiments indicate the formation of a 1:1 thorium:gluconate complex. The stability constant for this species, extrapolated to I=0 by using the SIT (specific interaction theory), has been determined to be log10Kº=−11.5 ± 0.6 for the reaction: Th4+ + GH4 − + 2H2O = Th(OH)2(GH2)− + 4H+. In the presence of ThO2·xH2O, sorption of the organic ligand onto solid surface seems to limit thorium dissolution.

Discrepancies in thorium oxide solubility values: study of attachment/detachment processes at the solid/solution interface

The solubility of thorium under oxide and/or hydroxide forms has been extensively studied for many years. Nevertheless, a large discrepancy in the solubility values is noticed in the literature. We study Th atom exchange between thorium oxide surfaces and various aqueous solutions (0.01 mol·L(-1) NaCl for 0.0 < pH < 5.2) to address this issue. By solid-state characterization [X-ray photoelectron spectroscopy (XPS), scanning electron microscopy, and atomic force microscopy], we determined that 80% of the XPS accessible near the surface region of sintered thorium oxide is represented by the less reactive ThO(2)(cr) grains. The remaining 20% corresponds to ThO(x)(OH)(y)(H(2)O)(z), which is largely associated with grain boundaries. Only the latter fraction is involved in solid/solution exchange mechanisms. Local conditions (thorium concentrations, pH values, etc.) in grain boundaries lead to an adjustment of the "local solubility constraints" and explain the thorium concentration measured in our experiments. For pH <5.2, the thorium concentration and pH gradient between the bulk solution and grain-boundary regions imply that the solubility values mainly depend on the availability and accessibility of ThO(x)(OH)(y)(H(2)O)(z). We have performed two solubility experiments with a (232)ThO(2)(cr) solid in a 0.01 mol·L(-1) NaCl solution for 300 days. In a first experiment, we measured (232)Th concentrations in dissolution experiments in order to determine the detachment rates of Th atoms from the solid surface. In a subsequent step, we added (229)Th to the solution in order to measure the surface attachment rate for dissolved Th atoms. This allowed an assessment of the net balance of Th atom exchange at the solid/solution interface. The empirical solubility data do not correspond to the thermodynamic bulk phase/solution equilibrium because measured solution concentrations are controlled by site-specific exchange mechanisms at the solid/solution interface. Therefore, for sparingly soluble solids, one needs to quantify site-specific surface attachment and detachment rates if one wants to assess solubility constraints.

Solubility of tetravalent actinides in alkaline CaCl2 solutions and formation of Ca4[An(OH)8]4+ complexes: A study of Np(IV) and Pu(IV) under reducing conditions and the systematic trend in the An(IV) series

The solubility of Np(IV) and Pu(IV) hydrous oxides was studied at 22±2 °C under reducing conditions in alkaline CaCl2 solutions. Redox conditions were adjusted either with 2 mM Na2S2O4 or additions of iron powder. In 2.0 and 4.5 M CaCl2 and pHc=11–12, the neptunium and plutonium concentrations increase with a slope of +4 (log[An] vs. pHc) as expected for the formation of the complex Ca4[An(OH)8]4+ recently identified for Th(IV). At CaCl2 concentrations ≤1.0 M this effect is negligible for both Np(IV) and Pu(IV). The conditional equilibrium constants log* K s,(4,1,8) for the reaction

An(OH)4(am) + 4 H2O + 4 Ca2+ ⇔ Ca4[An(OH)8]4+ + 4 H+

are evaluated with the SIT and Pitzer model using the parameters derived from analogous data for Th(IV) in 0.5–4.5 M CaCl2. The log* K°s,(4,1,8) values and the complex formation constants logβ°(4,1,8) follow systematic trends in the series Th(IV), Np(IV) and Pu(IV) which allows the estimation of the corresponding data for U(IV).

Discrepancies in thorium oxide solubility values: a new experimental approach to improve understanding of oxide surface at solid/solution interface

The solubility of ThO2(cr) was studied since many years but a large discrepancy in solubility values is noticed in the literature. The present work suggests that this discrepancy is related to differences in the surface properties of thorium oxide.

To understand the role of surface properties on solubility values, we conducted experiments with ThO2(cr) spheres with reproducable surface properties. Batch dissolution experiments were conducted in 0.01 M NaCl solution at pH=3.0 and 4.0 for periods of time up to 270 days. The solutions were spiked with229Th to determine precipitation (sorption) rates of thorium, while dissolution rates were determined by measuring232Th released from ThO2(cr) spheres. We assume that229Th atoms are exchanged only with active sites involved in Th-dissolution. Using229Th as local sensor of attachement and detachment processes at the ThO2(cr) surface under close-to-equilibrium conditions, allows to assess surface reactivity of the solid during solubility experiments.

Ion interaction (SIT) coefficients for the Th4+ ion and trace activity coefficients in NaClO4, NaNO3 and NaCl solution determined by solvent extraction with TBP

Trace activity coefficients of the Th4+ ion in dilute to concentrated NaClO4, NaNO3 and NaCl solutions ([H+]=0.01−0.02 M) have been determined at 22 °C from liquid-liquid phase equilibria with 10−50 vol.% TBP in n-dodecane. Using the specific ion interaction theory (SIT) to describe these equilibria as a function of the medium electrolyte concentration, the following ion interaction coefficients are calculated: ε(Th4+, ClO4 -)=0.70±0.06 kg/mol and ε(Th4+, NO3 -) = 0.31±0.12 kg/mol. The latter value differs considerably from ε(Th4+, NO3 -) = 0.11±0.02 kg/mol used in the NEA-TDB. The low distribution coefficients at m NaCl<3 mol/kg do not allow the linear SIT extrapolation to I=0, but the equilibrium constants in 2.5−5.0 m NaCl are compatible with the NEA-TDB value of ε(Th4+, Cl-)=0.25±0.03 kg/mol.

The SIT coefficients determined for the Th4+ ion follow the linear correlations between known values of ε(M Z+, NO3 -), ε(M Z+, Cl-) and ε(M Z+, ClO4 -) for non-complexed cations MZ+ with Z=1−4, including ε(Pu4+, ClO4 -)=0.82±0.07 kg/mol and ε(Pu4+, Cl-)=0.37±0.05 kg/mol calculated from data accepted in the NEA-TDB. The interaction coefficients in the series of the tetravalent actinide ions show a slight systematic dependence on the ionic radius.

Formation and hydrolysis of polynuclear Th(IV) complexes – a nano-electrospray mass-spectrometry study

Polynuclear hydroxide complexes play an important role for the hydrolysis of tetravalent thorium ions in aqueous solution, in particular for Th(IV) concentrations exceeding some [Th(IV)]=10−4 M. Consequently, these polymers must be considered when describing hydrolysis of Th(IV) or dissolution processes of Th(IV) solids. In the past, considerable efforts were made to obtain equilibrium formation constants of these polymers and different stoichiometries for dimers, tetramers and hexamers have been suggested. However, most information was obtained from indirect methods, in particular, from potentiometric titrations. In the present work, we present an approach of directly quantifying polymeric metal hydroxide complexes in solution. By nano-electrospray mass-spectrometry the degrees of polymerization, i.e. the numbers of Th4+ ions and the numbers of hydroxide ligands, and as a consequence, also the charges of the complexes are measured. All mono- and polynuclear species which are present in solution are quantified simultaneously down to species contributing less than 0.1% of the total [Th(IV)] concentration. Solutions of [Th(IV)]=6×10−6–10−1 M are investigated in HCl at [H+]=10−4–0.1 M. More than 30 different polymeric complexes are observed with the general trend of increasing number of hydroxide ligands with decreasing acidity. A surprising finding is the presence of the pentamer Th5(OH)y z +, which was not described in the literature before. With decreasing Th(IV) concentration the stability field of polymers narrows continuously until polymers can no longer be detected below [Th(IV)]=10−5 M.

Thorium reactions in borosilicate-glass/water systems

Studies were conducted on the dissolution of Th-doped borosilicate glass, a complex assemblage of 24 different elements in various proportions, in a wide range of pH values (0.5 to 12) and carbonate (as high as 6.2 m) and bicarbonate (as high as 1.0 m) concentrations, and as a function of time to determine whether the observed thorium concentrations exhibit an equilibrium phenomenon and to ascertain whether existing thermodynamic data can be used to interpret these results. Measurable Th concentrations were observed in either the very acidic (pH 0.5 to 4) solutions or alkaline solutions containing relatively high carbonate/bicarbonate concentrations. Steady state Th concentrations were reached over time from both the oversaturation and undersaturation directions, indicating that Th concentrations are controlled by an equilibrium solubility phenomenon. The XRD, EXAFS, and thermodynamic analyses of solubility data failed to definitively identify the nature of the solubility-controlling solid in this complex system; however, the data suggests that the solubility-controlling solid most likely involves the Th-silicate phase and that the observed Th concentrations are up to many orders of magnitude lower than in equilibrium with ThO2(am). The solubility study also showed that existing thermodynamic data for carbonato complexes of Th can be used to reliably predict Th concentrations in relatively dilute to concentrated carbonate/bicarbonate solutions.

Solubility and colloid formation of Th(IV) in concentrated NaCl and MgCl2solution

The solubility of crystalline ThO2(cr) and amorphous hydrated Th(IV) oxyhydroxide ThOn(OH)4-2n·xH2O(am) has been measured in dilute to concentrated NaCl and MgCl2solutions equilibrated with magnesium hydroxide or hydroxychloride at 22±2 °C. The contributions of colloids to the total thorium concentrations observed in both over- and undersaturation experiments with amorphous Th(IV) precipitates have been analysed by ultracentrifugation. The solubility increasing effect of long-time stable Th(IV) eigencolloids, previously investigated in 0.5 M NaCl solutions, is also observed in concentrated 5 M NaCl. Ionic strength and chloride concentration have no effect on the stability of these hydrophilic Th(IV) oxyhydroxide eigencolloids, which are the predominant species in solution. They cause relatively high total thorium concentration in neutral to alkaline steady state solutions, independent of ionic strength: log[Th]tot≈log[Th]coll=-6.3±0.5. In concentrated MgCl2solutions saturated with magnesium hydroxychloride colloids, the formation of pseudocolloids,i.e., Th(IV) sorbed onto Mg2(OH)3Cl·4H2O(coll), leads to a further increase of the total thorium concentration up to 10-5M. The present results are discussed with regard to maximum Th(IV) and Pu(IV) concentrations in performance assessment calculations.

Hydrolysis of plutonium(IV) in acidic solutions: no effect of hydrolysis on absorption-spectra of mononuclear hydroxide complexes

Tetravalent plutonium readily undergoes hydrolysis even in highly acidic aqueous solutions. In the past, many attempts were made to quantify hydrolysis species by means of optical absorption spectroscopy. In the present work solutions ranging from 10−5M to 10-2M (total Pu) concentration in 0.5 M HCl/NaCl (0.3 < pHc< 2.1) are carefully investigated by combining absorption-spectroscopy (UV-Vis, liquid core waveguide capillary) and laser-induced breakdown detection, with special emphasis on the limited solubility of Pu(IV). The results clearly indicate that all changes in the absorption spectra originate from the formation of Pu-polyspecies and colloids. The molar absorptivity of mononuclear Pu(IV) hydroxide complexes does not vary with increasing pHcand ongoing hydrolysis. The normalized absorption spectra of at least the first and the second hydroxide complex (Pu(OH)n4-nn= 1, 2) do not differ from those of the hydrated Pu4+ion.

Solubility of crystalline thorium dioxide

The solubility of thorium oxides of different crystallinity is investigated at 25°C by different experimental approaches. The dissolution of bulk crystalline ThO2(cr) is a very slow process and the Th(IV) concentrations measured after one year at pH 1-3 in 0.1 and 0.5M HCl-NaCl solutions do not represent equilibrium data. Coulometric titration of thorium nitrate solutions in the low pH range of 1.5-2.5 leads to the formation of microcrystalline ThO2·xH2O(mcr) particles which subsequently agglomerate to a precipitate. The solubility of this solid, in equilibrium with Th4+(aq), is measured from the oversaturation direction. The solubility product is determined to be log K´sp=-49.9±0.4 in 0.5M NaCl corresponding to log K°sp=-53.2±0.4 (converted to I=0 with the SIT). It is close to the thermochemical value for ThO2(cr) and about 6 orders of magnitude lower than that of X-ray amorphous Th(IV) hydroxide or hydrous oxide. The differences in the solubility products are discussed with regard to the particle size and compared with analogous data for U(IV), Np(IV) and Pu(IV).

Above the threshold of hydrolysis of Th4+ at pH>2.5, the dissolution of microcrystalline ThO2·xH2O(mcr) is found to be irreversible. In near-neutral to alkaline solutions, the measured thorium concentrations approach those of amorphous Th(OH)4(am). Similar results are obtained with crystalline ThO2(cr) in 0.5M NaCl-NaOH solutions. The solubility is not controlled by the bulk crystalline solid but by amorphous fractions on the surface.

Solubility of ThO2·xH2O(am) and the formation of ternary Th(IV) hydroxide-carbonate complexes in NaHCO3-Na2CO3 solutions containing 0−4 M NaCl

The solubility of ThO2· x H2O(am) is studied at I=0.1−4.0 M (NaHCO3-Na2CO3-NaCl) and 22 °C. Several sets of closed system experiments are performed at total carbonate concentrations of C tot=[HCO3 -]+[CO3 2-]=0.02 and 0.1 M in the range pHc = 8−11. In our recent study at I=0.5 M, the simultaneous evaluation of solubility data at widely varied pHc and carbonate concentrations has shown that Th(OH)(CO3)4 5-, Th(OH)2(CO3)2 2- and Th(OH)4(CO3)2- are the most important ternary complexes. The present results at I=0.1−4.0 M are used to describe the ionic strength dependence of the equilibrium constants logK s, 1yz (for the reactions Th(OH)4(am)+zCO3 2-↔Th(OH) y (CO3) z 4-y-2z +(4-y)OH-) with the SIT. EXAFS measurements in 0.1 and 1.0 M carbonate solutions support the calculated speciation. They show clearly different spectra for the predominant species Th(OH)(CO3)4 5- and Th(CO3)5 6-, respectively.

Solubility of Zr(IV), Th(IV) and Pu(IV) hydrous oxides in CaCl2solutions and the formation of ternary Ca-M(IV)-OH complexes

The solubility of Zr(IV), Th(IV) and Pu(IV) hydrous oxides is investigated at 22±2 °C in alkaline 0.1−4.5 M CaCl2solutions. Further studies are performed with Zr(IV) over the entire pH range in NaCl and CaCl2media, and with Zr(IV) and Th(IV) in alkaline Ca(ClO4)2solutions. The comparison of Zr(IV) data in different ionic media (NaCl, NaClO4, CaCl2and Ca(ClO4)2) of similar ionic strength shows that the solubility in the acidic and neutral pH range is not affected by strong interactions between the aqueous M(IV) species and the medium ions. However, in alkaline CaCl2and Ca(ClO4)2solutions the formation of ternary Ca-M(IV)-OH complexes causes unexpectedly high solubilities of Zr(IV) at pHc=10−12 and [Ca2+]>0.05 M and of Th(IV) at pHc=11−12 and [Ca2+]>0.5 M. The dependence of the Zr(IV) and Th(IV) solubilities on the H+and CaCl2concentrations shows that the complexes Zr(OH)62−and Th(OH)84−with an unusual large number of OH−ligands are stabilized by the formation of associates or ion pairs with Ca2+ions. The SIT is used to derive equilibrium constants at zero ionic strength for the complexes Zr(OH)62−(in calcium-free solutions), Ca2[Zr(OH)6]2+, Ca3[Zr(OH)6]4+and Ca4[Th(OH)8]4+. In analogous studies with Pu(IV) hydrous oxide, the solubility increasing effect of ternary complex formation with Ca2+ions is only observed at CaCl2concentrations above 2 M.